The present invention relates to an apparatus for obtaining the information on a blood stream within a body under examination at a certain depth below the skin surface by utilizing ultrasonic Doppler imaging, and more particularly to such an ultrasonic blood stream observing apparatus capable of obtaining highly accurate blood stream information at a desired depth within a body being examined.
Ultrasonic Doppler imaging is based on the principle that when an ultrasonic wave is reflected by a moving object, the reflection is subjected to a frequency shift proportional to the speed of movement of the moving body. More specifically, ultrasonic rate pulses or a continuous ultrasonic wave is transmitted into a living body, and the echo has a frequency shift due to its phase change according to the Doppler effect. The frequency shift is utilized to obtain the information on movement of the moving object at a depth where the echo is produced. Such ulstrasonic Doppler imaging is effective in gaining blood stream information which indicates various blood stream conditions in a certain position within a living body, such for example as the direction of the blood stream, whether the blood stream is disturbed or smooth, the pattern of the blood stream, and the absolute value of the speed of flow of the blood stream.
FIG. 9 of the accompanying drawings shows the manner in which ultrasonic Doppler imaging is carried out. When an ultrasonic beam having a frequency f.sub.0 is transmitted from a transducer into a living body under examination at an incident angle .theta. with respect to a blood stream in a blood vessel, the frequency f.sub.0 is shifted upon being reflected from the moving object or blood stream which flows at a speed v. Assuming that the shifted frequency, which is the frequency of a received signal, is expressed by f.sub.0 ', the frequencies f.sub.0, f.sub.0 ' have the following relationship (1): ##EQU1## where C is the speed of travel of the ultrasonic beam in the living body.
Since the speed of sound within the living body is sufficiently higher than the blood stream speed v, the frequency shift fd (=f.sub.0 '-f.sub.0) can approximately be given by the following equation (2): ##EQU2##
Ordinary ultrasonic blood stream observing apparatus display the above Doppler frequency shift fd.
In the pulsed Doppler imaging process in which high-frequency pulses are employed as a transmitted signal, there are transmitted spectra present at increments of a pulse repetition frequency (rate pulse) f.sub.PRF from the central frequency f.sub.0. Therefore, a signal is received with respect to each of the spectra, and the Doppler signal at f.sub.0 +nf.sub.PRF can be expressed by the following equation (3) which is derived by modifying the above equation (2): ##EQU3## where n is an integer.
According to the pulsed Doppler imaging process, the Doppler component fdn in each f.sub.0 +nf.sub.PRF can be selected as desired within an allowable range of S/N ratio. In general, which Doppler component is to be extracted can be determined by a reference frequency f.sub.R used when demodulating the Doppler component from a high-frequency range into an audible range. The obtained Doppler signal is expressed by the equation (4): ##EQU4##
The reference frequency f.sub.R should preferably be set to a frequency with the highest S/N ratio in the received spectrum. It is necessary that the pulse repetition frequency f.sub.PRF and the reference frequency f.sub.R be completely in synchronism with each other since if the pulse repetition frequency f.sub.PRF and the reference frequency f.sub.R were brought out of phase in time, such an out-of-phase condition would be detected as a Doppler frequency shift resulting in unwanted noise.
The pulse repetition frequency f.sub.PRF and the reference frequency f.sub.R have heretofore been generated as shown in FIG. 10 of the accompanying drawings. An oscillator 1 produces a reference clock signal f.sub.B, which is frequency-divided at 1/m (m is an integer) into a pulse repetition frequency f.sub.PRF (=f.sub.B /m) by a frequency divider 2. The reference clock signal f.sub.B from the oscillator 1 is also frequency-divided at 1/n (n is an integer) into a reference frequency f.sub.R (=f.sub.B /n) by a frequency divider 3.
As described above, the reference frequency f.sub.R should preferably be set to a frequency with the highest S/N ratio in the received spectrum. Because the reference frequency f.sub.R is produced by frequency-dividing the reference clock signal f.sub.B with the frequency divider 3 as shown in FIG. 10, the reference clock signal f.sub.B should be of a high frequency in order to vary the reference frequency f.sub.R in small steps, with the results that the circuit is large and noise is increased.
In order to achieve full synchronism between the pulse repetition frequency f.sub.PRF and the reference frequency f.sub.R, the following relationship must be met: EQU f.sub.R /f.sub.PRF =m/n=integer
This imposes limitations on the selection of m, n, i.e., the pulse repetition frequency f.sub.PRF and the reference frequency f.sub.R.
Inasmuch as the pulse repetition frequency f.sub.PRF and the reference frequency f.sub.R have been generated by frequency-dividing the reference clock signal in the conventional ultrasonic blood stream observing apparatus, the reference frequency f.sub.R cannot be set to a desired value- Moreover, it has been difficult to bring the pulse repetition frequency f.sub.PRF and the reference frequency f.sub.R into synchronism with each other. As a consequence, blood stream observations at desired depths cannot be effected with high accuracy.